In a thermal printer, and other web transport systems, it is desirable to accurately position the web laterally in its plane. One such web center-guiding apparatus is disclosed in U.S. Pat. No. 4,889,269 and illustrated in prior art FIG. 8 wherein an air bar 1 supports and laterally center-guides a thin web 2 of material. The air bar 1 is mounted on a housing 3 via an end cap 4 and a bolt 5. The air bar 1 has a center ring 6 with apertures 7. Pressurized air travels through a passageway 8 in the housing 3 to the apertures 7 to hold the web 2 off the center ring 6 to provide a bearing surface. There are also guide apertures 9 for laterally center-guiding the web 2. U.S. Pat. No. 4,288,015 similarly discloses a contactless web turning guide which also uses apertures or air jets. In a work environment, such as a thermal printer, for example, operation of the apertures may be affected by dust or other debris thereby thwarting the center-guiding function.
The task of lateral positioning is often complicated by additional requirements such as using a common center line for various widths of webs each having some width tolerances. Another factor is maintaining the same path for several passes through the control portions of the system. U.S. Pat. No. 4,037,769 discloses a guide roller assembly that adjusts to accommodate different widths of material to maintain a common centerline. Unfortunately, the adjustment must be accomplished manually by turning knobs and screws. U.S. Pat. No. 3,261,525 discloses a tape guide which must also be adjusted manually to laterally center the tape about a drum. German Patent No. 2,533,769 discloses an adjusting device for a tire cord strip moving longitudinally in a reference plane. Two carriages are supported on a frame so that they can move in the transverse direction and are connected with a central device via a coupling element such that the two carriages can be moved simultaneously toward and from one another relative to the reference plane. Both carriages are permanently in contact with the edges of the strip for adjustment. While there is automatic adjustment, the apparatus is bulky with numerous parts for handling thick, heavy tire cord strips confined to a flat plane. Accordingly, it will be appreciated that it would be highly desirable to have a web guidance mechanism that is easy to manufacture, simple to install, and requires no manual adjustments for centering use with webs of various widths and width tolerances.